The present invention relates to an active-type trigonometric rangefinding optical system in which light is projected to an object and the reflected light from the object is received by a rangefinder.
Active-type trigonometric rangefinders used on, for example, modern compact still cameras project light such as near-infrared light to a subject and receive the reflected light from the subject by means of a photoconversion element. The reflected light falls upon the photoconversion element over a certain width. This width on the photoconversion element causes errors in measured subject distances.
In an attempt at avoiding measurement errors, there have been proposed various optical systems for use with active-type rangefinders. One such optical system disclosed in, for example, Japanese Patent Publ. No. 61-20,884 utilizes a single aspherical lens therein for correcting aspherical and coma aberrations so as to form a sharply focused spot on incident light on the photoconversion element. Another optical system disclosed in Japanese Patent Unexam. Publ. No. 62-47,613 utilizes double aspherical lenses so as deliberately to produce an increased curvature of field in order to form a relatively small light spot projected onto a subject within an effective subject distance measurement range even though the light spot is out of focus on the subject and is reflected from the subject and falls on the photoconversion element.
In the optical system disclosed in the former publication Pat. No. 61-20,841, because of a small curvature of field in a meridianal surface produced by the optical system, if the light projecting lens system is focused on a subject at an infinite distance, it is hard to focus it on subjects at relatively short distances, resulting in an inaccurate subject distance measurement.
On the other hand, in the optical system disclosed in the latter publication Pat. No. 62-47,613, although the above described disadvantage associated with the optical system disclosed in the former publication is overcome, there is still the disadvantage that the optical system produces curvature of field, which cannot be disregarded not only in a meridianal image surface but also in a sagittal image surface because of the Petzval sum being increased in order to produce a significant curvature of field.
If one incorporates the optical system taught by the above described publication Pat. No. 62-47,613 into a rangefinding device such as is disclosed in Japanese Patent Unexam. Publ. No. 61-246,726, which is of the type projecting light in the form of a line extending in a direction perpendicular to the direction of a base line to a subject in order to measure an extensive area in the field of the rangefinder, the distance of a plurality of subjects within the same subject distance zone can be measured. In such a case, there are light spots incident upon the photoconversion element at different positions which may not be equally in focus.